The United States House of Representatives is voting on Thursday on whether to hold Steve Bannon, a long-time ally and aide to former President Donald Trump, in contempt of Congress after he defied a subpoena from a committee investigating the violent January 6 Capitol insurrection.

That committee has vowed to move swiftly and forcefully to punish anyone who will not cooperate with the probe. But it’s likely up to the Justice Department, and the courts, to determine what happens next.

If the House vote succeeds, as is expected, there’s still considerable uncertainty about whether the Justice Department will prosecute Bannon, despite Democratic demands for action.

The outcome could determine not only the effectiveness of the House investigation but also the strength of Congress’ power to call witnesses and demand information — factors that will certainly be weighing on Justice officials as they determine whether to move forward.

While the department has historically been reluctant to use its prosecution power against witnesses found in contempt of Congress, the circumstances are exceptional as lawmakers investigate the worst attack on the US Capitol in two centuries.

To emphasise the committee’s unity in holding Bannon accountable, the panel’s Democratic chairman, Bennie Thompson, will lead the debate on the bill along with Republican Liz Cheney of Wyoming, one of two Republicans on the committee — a rare show of bipartisanship on the House floor.

Still, most House Republicans are expected to vote against the contempt measure, despite the potential consequences for the institution.

If Congress cannot perform its oversight job, the message sent to “the general public is these subpoenas are a joke,” said Stephen Saltzburg, a George Washington University law professor and former Justice Department official.

He said if attorney general Merrick Garland, a former federal judge whom Saltzburg regards “as one of the most nonpartisan people I know,” does not authorise a prosecution, “he’s going to be letting the Constitution, it seems to me, be placed in jeopardy. And it’s way too important for him to let that happen.”

Democrats are pressuring Justice to take the case, arguing that nothing less than democracy is on the line.

“The stakes are enormous,” said Maryland representative Jamie Raskin, a member of the panel. “The Congress of the United States under Article One has the power to investigate in order to inform our deliberations about how to legislate going forward. That’s what this is about.”

Still, prosecution is not a given. Assuming his post after a turbulent Trump era, Garland has prioritised restoring what he has called “the norms” of the department.

On his first day, he told rank-and-file prosecutors that they should be focused on equal justice and not feel pressure to protect the president’s allies or to attack his enemies. He has repeatedly said political considerations should not play a role in any decisions.

Even if the department does decide to prosecute, the case could take years to play out — potentially pushing past the 2022 election when Republicans could win control of the House and end the investigation.

And if they do not prosecute, then the House will likely find another route. A House-authorised civil lawsuit could also take years but force Bannon and any other witnesses to defend themselves in court.

Another option available to Congress would be to try to imprison defiant witnesses — an unlikely, if not outlandish, scenario. Called “inherent contempt”, the process was used in the country’s early years but hasn’t been employed in almost a century.